This invention relates to the field of machining of materials and, more particularly, to ultrasonic machining and a method of control of ultrasonic machining apparatus. This invention is the result of a contract with the Department of Energy (Contract No. W-7405-ENG-36).
Ultrasonic machining is the removal of material from a workpiece by particles of an abrasive substance which vibrate in a generally vertical direction above and in contact with the workpiece. Movement of the abrasive particles, which are often termed grit, is caused by a tool which oscillates in a vertical direction at about 20,000 Hz, though frequencies up to 40 kHz have been used. The amplitude of the oscillation, or vibration, of the tool is adjustable and may range from less than 0.001 in. to 0.025 in. (0.0254-0.635 mm) with 0.007 in. (0.178 mm) being a typical value. The tool is mounted above but not touching the workpiece and the abrasive particles are in a slurry which is supplied to a gap between the tool and workpiece and flows through the gap.
When a workpiece is stationary, the tool makes a hole in the workpiece having a configuration which is a mirror image of the tool. The tool does not normally rotate and therefore cavities having complex shapes may be machined (though rotating tools have been used in ultrasonic machining).
The accuracy of this machining process depends on factors including the size of the tool, rigidity of the equipment carrying the tool, temperature of the slurry, and size of the abrasive particles. Accuracy of plus or minus 0.001 in. (0.025 mm) can usually be attained and plus or minus 0.0002 in. (0.005 mm) can be achieved with special care. Ultrasonic machining is used primarily on hard and brittle materials, such as ceramics. However, it can be used to machine ductile materials, but with more difficulty. Ultrasonic machining is particularly well suited to the production of relatively shallow irregular cavities and is one of the few machining processes suitable for machining fragile material. The main disadvantages of ultrasonic machining are a low material removal rate and high cost. Thus, it tends to be used only in those applications where other methods of machining fail or produce poor results.
The tool moves in a vertical direction and the vertical gap between the lower face of the tool and the workpiece during machining usually ranges from about 0.0005 to about 0.0025 in. (0.0127-0.0635 mm). It is desirable that the tool be relatively soft but tough. It can be said that the tool captures and throws the grit at the workpiece. Metals that are extremely ductile, such as copper, brass, and aluminum yield a short tool life, while tough materials such as low carbon steel, stainless steels, and molybdenum give good tool life.
The slurry usually contains about 30 to 60% of abrasive by volume in water. Other liquids may be used in place of water, but water is convenient and cheap. The abrasives most used in ultrasonic machining are industrial diamond dust, silicon carbide, boron carbide, and aluminum oxide. Slurry is discharged through a nozzle located adjacent to the cavity being machined in the workpiece. Slurry flowing away from cavity is collected and recycled. It is often necessary to cool the slurry as it picks up a considerable amount of heat. As in conventional grinding, finer abrasives yield smoother finishes, though finer grit usually cuts at a slower speed. Grit sizes from about 200 to about 400 (0.0026-0.0009 in. or 66-23 microns) are used for rough cutting and sizes from about 800 to about 1000 (0.00044-0.00032 in. or 1114 8 microns) are used for finishing. Fresh abrasive cuts better than grit which has been in use for a period of time so grit should be added to the slurry from time to time and the slurry should be completely replaced at intervals. Also, the concentration of material removed from the tool and the workpiece builds up in the slurry.
The speed at which parts can be machined and the quality of the part produced is highly dependent upon the tool feed rate, that is, the rate at which the tool is moved downward toward the workpiece. If the tool feed rate is too rapid, tool wear will be high and the risk of chipping or cracking the brittle workpieces on which ultrasonic machining is used will be great. If the tool contacts the workpiece, there will be damage to both to the extent that both will likely have to be scrapped.